1. Field of the Invention
This invention relates, in general, to optical systems and, more specifically, to illumination of documents in line scanners.
2. Description of the Prior Art
Document scanners are used to scan across the surface of a document and input electrical information which represents optical indicia located on the document. Basic document scanning systems can take several different forms depending upon the type of illumination and the type of pickup, or sensor, used to detect reflected light from the document. Line scanners illuminate a complete line on the document and distinguish between elements, or pixels, on the line by a suitable sensor array, such as a linear charge coupled device CCD). In some line scanners, the illuminated line is no higher than the width of a pixel being scanned on the line. In other line scanners, the illumination line can be larger than the pixel size because only a portion of the line is focused onto a sensor area for detection, Regardless of the particular arrangement, there is a need in document line scanners to produce a predictable and constant line of continuous light across the document to provide the reflected light needed for the sensors.
Bundles of optical fibers have been used according to one prior art arrangement for providing the illumination line needed in scanners. One end of the fibers in a group, or bundle, is positioned near to a source of light energy, such as an incandescent lamp. The other ends of the fibers are spread out in the shape of the line so that the light leaving the fibers is of the desired shape. While this arrangement provides the necessary shape and dimensions, the cost of such an optical fiber bundle can be a considerable portion of the entire scanning apparatus. In the interest of economy, it is desirable to provide the line illumination with a much more economical device.
U.S. Pat. No. 4,520,757, issued on Mar. 5, 1985, discloses a light beam distributor with some similarities to the device taught by the present invention. The patented device divides a coherent laser beam into several independent and separated beams for the purpose of passing the beams through a multi-channel light modulating element and eventually irradiating a photosensitive member. Although several spots of light are produced at the same time, the result is not in a continuous line. The light beam distributor disclosed in the patent uses a reflecting coating on one side and a semi-reflecting, or transmitting-reflecting, coating on another side. This second coating is changed in thickness to equalize the attenuation rates of the outgoing light beams. This device also does not have other sides or faces oriented in a particular non-parallel fashion to condense the entering light by total internal reflection and produce a line having a height which is smaller than the original beam diameter.
For the reasons indicated, it is desirable, and an object of this invention, to provide an economical illumination device for use in document line scanners.